1. Field of the Invention
The present invention relates to a control circuit for a vehicle safety device which starts the vehicle safety device.
2. Description of the Related Art
FIG. 4 illustrates a conventional control circuit for vehicle safety device such as that disclosed, for example, in Japanese Patent Laid-Open No. 3-57748. Through a well-known safety switch 7, ignition squibs 5 and 6, field-effect transistors 11 and 21, and current-limiting circuits 15 and 25 are connected in series with power supply section 4. Ignition squibs 5 and 6 function respectively as starting means for a driver's seat air bag A and starting means for air bag B disposed near the passenger seat next to the driver. A current-flow controlling section 3 is connected to a sensor 2 which detects acceleration of the vehicle (not illustrated) and outputs an acceleration signal KS. The current-flow controlling section 3 performs predetermined processings on acceleration signal KS, judges whether or not the vehicle has collided, and, when it has judged that a vehicle collision occurred, outputs starting signals T1 and T2 respectively to the gates of transistors 11 and 21 respectively through resistances 14 and 24.
Current-limiting circuits 15 and 25 are composed of, respectively, current detecting resistance 12 and transistor 13, and current detecting resistance 22 and transistor 23. Detection voltages Va and Vb, developed respectively across current detecting resistances 12 and 22, are applied, respectively, between the bases and emitters of transistors 13 and 23, with the collectors of transistors 13 and 23 connected respectively to the gates of transistors 11 and 21.
When the speed of a vehicle exceeds a predetermined value, the safety switch 7 is closed. With the switch in a closed state, starting signals T1 and T2, output from the current-controlling section 3, turns on, respectively, field-effect transistors 11 and 21. This causes current to flow from the power supply section 4 to ignition squibs 5 and 6. Here, the currents which flow from the power supply section 4 into the ignition squibs 5 and 6, are maintained at predetermined values, respectively, by the action of current-controlling circuits 15 and 25. The circuit constants of current-controlling circuits 15 and 25 are preset so that sufficient current flows through ignition squibs 5 and 6 which allows inflation of air bags A and B when transistors 11 and 21 are on. This prevents an excess of ignition current flow.
The power supply section 4 contains a battery 41, connected to safety switch 7 through a diode 46, and a high-capacity capacitor 44 connected directly to safety switch 7. A booster circuit 42 for supplying charging voltage is connected to the capacitor 44 through a diode 43. Upon vehicle collision, ignition current is supplied from battery 41 and capacitor 44 to ignition squibs 5 and 6. It is to be noted that even when disconnection at the terminals of battery 41 or the like occurs upon collision, electrical energy accumulated on the plates of the capacitor allows ignition current to be supplied to ignition squibs 5 and 6,
In such a control circuit for a vehicle safety device, as shown in FIG. 5, while ignition squibs 5 and 6 are placed respectively near the driver's seat air bag and the air bag for the passenger's seat next to the driver, the typical battery 41 and sensor 2 and control unit 1 which accommodate other component parts are placed in the engine room of the vehicle. This means that the control unit 1 and the ignition squibs 5 and 6 are connected through long wiring lines L, called a harness. Consequently, when the wiring lines L are short-circuited with the vehicle body due to vehicle collision, deterioration of the line L, or the like, the line L may be electrically grounded. For example, when the point adjacent to the ignition squib with a high potential, point 1 (P1), in FIG. 4 is short-circuited with the vehicle body, the vehicle speed increases beyond a predetermined value, which closes the safety switch 7. When this happens, the positive terminal of battery 41 is in the same condition as when it is short-circuited with ground, so that an excessive current flows from battery 41 to safety switch 7 through wiring lines L. This may result in such hazards as fire. On the other hand, when the point adjacent to ignition squib 6 with a low potential, point 2 (P2), is short-circuited, even though there is no starting signal T1 output from current-flow controlling section 3, current flows through ignition squib 6, which accidently explodes air bag A.